From hitherto, heat-sensitive recording materials that use a color forming reaction through heat between a leuco dye and a color developer have been well known. Such a heat-sensitive recording material is relatively inexpensive, and moreover the recording apparatus is compact and relatively easy to maintain. Heat-sensitive recording materials have thus come to be used not only as recording media for facsimiles, various calculating machines and so on, but also as recording media used in printers of equipment for medical diagnosis such as MRI diagnostic imaging and X-ray diagnostic imaging, and recording media for CAD plotters and the like.
However, with such a heat-sensitive recording material that uses a leuco dye and a color developer, there is a drawback that, upon the amount of moisture in the heat-sensitive recording layer changing accompanying a change in the humidity of the environment in which the recording material is used, the recording sensitivity fluctuates, and as a result the recording density fluctuates, and in particular it is difficult to obtain constant recording density in a halftone region. This drawback is particularly marked in the case of using a support such as a synthetic resin film.
For a heat-sensitive recording material, it is desirable for the recording sensitivity to always be constant, not fluctuating due to differences in the humidity of the usage environment with season, region, country or the like. In particular, for a heat-sensitive recording material used for medical diagnosis, there are strong demands for images on a par with those using a silver salt system to be obtained, for recording to be possible with multiple gradient steps, and for the recording density in the halftone region to be constant, not being affected by changes in the humidity of the environment. There are thus strong demands for a heat-sensitive recording material for which the above drawback is improved upon.
4,4′-cyclohexylidenediphenol has been known from long ago as a color developer used in heat-sensitive recording materials (see patent document 1). Moreover, there has also been proposed a method in which this color developer is used, so as to obtain a heat-sensitive recording material having little background fogging, and having a large difference in transmission density between recorded portions and non-recorded portions (see patent document 2).
Moreover, 4,4′-bis(N-p-tolylsulfonylaminocarbonylamino)diphenylmethane is also known as a color developer, and there has also been proposed a method in which this color developer is used so as to obtain a heat-sensitive recording material for which the oil resistance and plasticizer resistance of recorded images are improved, and hence the long-term stability of recorded images is improved (see patent document 3).
Moreover, to improve the storability for recorded portions of a heat-sensitive recording material, there have been proposed, for example, a method in which there are used composite particles obtained by making a leuco dye and a hydrophobic organic solvent into microcapsules using a wall material made of a hydrophobic resin (see patent documents 4 and 5), a method in which there are used composite particles obtained by polymerizing an acrylic hydrophobic resin on the surface of a leuco dye (see patent document 6), and a method in which there are used composite particles obtained by making a leuco dye be contained in a hydrophobic resin (see patent documents 7 and 8).
Patent document 1: Japanese Examined Patent Publication No. 45-14039 (page 4)
Patent document 2: Japanese Unexamined Patent Publication No. 9-76641 (claim 1)
Patent document 3: Japanese Unexamined Patent Publication No. 5-147357 (example 1)
Patent document 4: Japanese Unexamined Patent Publication No. 60-244594 (claim 1)
Patent document 5: Japanese Unexamined Patent Publication No. 61-86283 (example 1)
Patent document 6: Japanese Unexamined Patent Publication No. 2000-158822 (claims 1 and 7)
Patent document 7: Japanese Unexamined Patent Publication No. 9-263057 (claim 1)
Patent document 8: Japanese Unexamined Patent Publication No. 2003-266951 (claim 1).